weird noise coming from 89 560 SEL
 

This noise only occurs when I'm on the freeway. Here's what happens: After a few minutes driving on the freeway, I will hear this high pitched whistling sound for 5 to 10 seconds coming from the front of the car, probably somewhere in the engine compartment. This event will occur about every 5 to 7 minutes. If anyone can be of assistance I will be very grateful.

Thanks in advance.

Could it be the belt for the AC compressor when it starts up? They cycle when driving in high speeds.

if it is the a/c belt, how expensive is it to fix it and is it easy? or do i have to replace the whole compressor?